How can the Levels compositor node be used?
This node will output a single float value based on input image.




Mean is the average value of all image pixels in specified channel (combined, red, green, blue, luminance). It tells you how dark or bright the image is and can be used as such for setups that depend on how is input "bright" or "dark".
The Std Dev is how much those pixel values differ from the mean. A low standard deviation indicates that the pixel values tend to be very close to the mean. A high standard deviation indicates that the values are spread out over a large range of values.


The visualization of such data is just a grey rectangle.